1. Field of the Invention (Technical Field)
The present invention relates to a system for spreading a multi-filament bundle of fibers into individual fiber strands, particularly for spreading the bundle into individual strands for various subsequent treatments and coating processes. The invention is particularly well suited for various carbon fiber applications, including prepreg processing, and can operate over a wide range of fiber speeds and bundle spread range.
2. Background Art
Many industrially useful fibers are manufactured, or transported, or stored, in collective bundles. A number of industrial and manufacturing processes require that a bundle of fibers be spread to expose the exterior surfaces of the individual fibers. Commonly in such processes, it is needful to separate a bundle or "tow" of collected fibers into individual fibers so that the individual fibers can be subjected to further processing, for example to be coated with a substance, incorporated into a composite component, and the like.
Various methods have been developed for spreading fiber bundles into their constituent strands. Known methods typically involve vibration, pneumatics, the use of rigid barrel-shaped rollers, or electrostatic charging of the fiber bundle.
In U.S. Pat. No. 5,101,542, for example, a fiber separator includes a separating roller composed of a plurality of bulged, barrel-shaped rollers having their respective axes arranged in a circle. The rollers force the fiber bundle to separate into individual fibers at a rigid bulged surface in such a manner that the fiber bundle is flattened along the bulged surface. It is indicated that the running speed of the fiber bundle is slow, thus limiting the utility of the spreader to applications tolerating relatively slow process speeds. Also, it is likely that such a system could damage delicate fiber bundles.
U.S. Pat. No. 3,704,485 describes a method and apparatus for spreading a tow of fibers using a speaker or other gas vibrator adjacent to the fibers. The tow of fibers passes over the speaker under low tension, and is spread by the sound waves created by the speaker. With this method, the tow of fibers is unrestrained and can be damaged. Furthermore, controlling the fiber spacing within the spread fiber bundle is difficult.
The device described in U.S. Pat. No. 5,042,111 appears to operate on the principle that a pulsating flow of energy in a gaseous medium spreads a collimated fiber tow into its individual filaments. The source of energy is a vibrating cone or diaphragm of a speaker, or other vibrator, which oscillates at a predetermined frequency and amplitude. The acoustic energy provided by the speaker enables the fiber tow to spread.
Cited examples in U.S. Pat. No. 5,042,111 do not indicate speeds greater than 3 cm/sec (5.9 ft/min). A similar spreader, discussed in U.S. Pat. No. 5,128,199, and having the same inventors as U.S. Pat. No. 5,042,111 also uses acoustic vibration, preferably an acoustic speaker.
U.S. Pat. No. 4,994,303 describes a method of spreading yarn fibers with spreader bars. The spreader bars change the cross-sectional configuration of the yam from a tightly bundled and compressed round shape to a flat, ribbon-like configuration. The yarn fibers are drawn over and under the spreader bars at sharp angles under tension. This system is likely to damage delicate fiber bundles such as un-sized carbon fiber. The method also does not appear to have the ability to control the spread width.
Described in U.S. Pat. No. 4,799,985 is a gas banding jet for spreading fiber tows. The banding jet consists of a gas box into which compressed air or another gas is fed through an adjustable gas metering means. One, or more than one, gas exit ports are provided to cause gas from within the gas box to impinge in a generally perpendicular fashion upon the fiber tow that passes across the exit ports.
U.S. Pat. No. 4,799,985 teaches a fiber comb having a plurality of spaced-apart fingers that act to separately maintain the various fine yams of a thermoplastic polymeric fibers, while U.S. Pat. No. 5,057,338 describes a method for spreading filamentary material using vacuum pressure.
U.S. Pat. No. 3,795,944 discloses a process and apparatus for pneumatically spreading thin graphite or other carbon filaments from a tow bundle to form a sheet or tape in which the filaments are maintained parallel. The process includes passing the tow through at least one slot venturi spreader in which the tow is pulled through the spreader in a direction opposite the primary air flow through the venturi. A process that operates on the same general principle is described in U.S. Pat. No. 3,873,389 which is a division of application Ser. No. 205,878, filed Dec. 8, 1971, now U.S. Pat. No. 3,795,944.
Using air, vacuum, or a pressurized gas to separate fiber bundles has limitations, and is not well suited for delicate fibers, particularly when operating at relatively high speeds. Adjustment and control of the spread width with such devices can be difficult.
U.S. Pat. No. 5,094,883 discusses a method of spreading towpregs whereby a tow is first woven through a series of three cylindrical rollers, to impart a constant tension on and to reduce the vibration of the tow filaments. The tow then enters an air comb where forced air further spreads the filaments.
U.S. Pat. No. 5,200,620, describes a fiber spreader that includes a thin electrically grounded support sheet for supporting a layer of fibers on one surface thereof. The fibers supported on the support sheet are guided by a plurality of guide rollers past a corona discharge region, where the fibers are spread electrostatically. Although speed limitations of the corona discharge device are not discussed in U.S. Pat. No. 5,200,620 it is anticipated that maintaining a desired spread at high speeds would be very difficult to achieve. Other corona discharge devices are described in U.S. Pat. No. 4,081,856, U.S. Pat. No. 3,967,118, U.S. Pat. No. 3,456,156 and U.S. Pat. No. 4,999,733.
Against the foregoing background, the present invention was developed. The inventive apparatus and process provide an alternative means and method for spreading multi-filament bundles of fibers without the need for pneumatics, vacuum pressure, electrical charge, vibration, or specially shaped rigid rollers, such as crowned or barrel rollers. The invention imparts minimal damage to the fibers, thereby allowing its use with delicate fibers. The inventive spreader is adjustable, which allows for good control of the spread width, and can operate from very low speeds (i.e. fiber bundle speeds less than 10 ft/min) to high speeds (i.e. fiber bundle speeds in excess of 300 ft/min). Several spreaders can be placed in series to achieve a spread that is many times the incoming width for the manufacture of thin, low areal density composite tapes.